In load handling devices adapted for shallow water barge mounted bucket dredge work, the load handling devices have generally consisted of track mounted devices operating from the deck of a barge or the upper frame or cab of a track mounted device mounted on a fixed tub on the deck of a barge. Ordinarily, whether track mounted or tub mounted on a barge, the mounting arrangement for such upper frame or cab consists of a turntable base having a plurality of house rollers and hook rollers mounted thereon and supported on a roller pad, and, if track mounted only, a shaft extending vertically downward from the cab through the turntable base to the crawler for delivering power from the power plant located on the turntable base to the crawler treads to propel the load handling device, or if tub mounted on a barge, a center pin for counteracting the turning moments resulting from loading on the extended boom. The vertically extending shaft or center pin also define the vertical axis about which the turntable base rotates. Further, such load handling devices are provided with swing mechanisms, including swing motors which are operatively connected to a swing gear by way of associated gears, transmissions, shafts, clutches, and the like to effect swing motion of the cab relative to the tracked vehicle frame or the tub.
As track mounted devices, load limits are not exceeded since the tracks will rise if a load limiting condition is reached. However, when a track mounted load handling device is mounted on a fixed tub on the deck of a barge, since the loading is outside the center of gravity of the load handling device, the design limits of the center pin, the house roller, and the hook roller are exceeded if the load limits are reached. Exceeding the design limits produces safety hazards and excessive wear not only on the center pin, house roller, and hook roller, but also on the swing mechanism, particularly, since the diameter of the house roller ring is limited by the width of the tracked vehicle frame. With such short house roller ring diameters and such extremely high load stresses prior tub mounted load handling devices either had high downtime and high maintenance costs or were heavily reinforced which added excess weight to the device thereby increasing the duty cycle of the equipment of producing excessive backlash in the swing mechanism.
In the past, the desirability to increase the house roller ring diameter necessitated machining a new plate to close tolerances and adding rollers thereto which, although, reducing stresses, did not reduce downtime due to wear on the house rollers and roller pad and, in effect, considerably increased the cost of the load handling device.